The ever increasing demand for cryogenic gases has led to a search for improved processes for separating the respective components of various gaseous mixtures, including air. Considerable investigation is being conducted in the area of semi-permeable polymeric membranes wherein such membranes exhibit selectivity toward the passage of one or more components of the gaseous mixture through the membrane.
Commercial applications for gas separation devices based on polymeric materials rely, in part, on maximizing the overall gas flux through the membrane. T. H. Kim, et al.. J. Appl. Poly. Sci., 34 1767 (1987), report that membrane gas flux is related to the average space between the polymer chains. In addition, the investigators indicate that the density of the polymer is also related to the overall gas flux.
The success of commercial applications depends upon the identification of polymers having very high flux and good thermo-mechanical properties. High overall flux values are typically exhibited in polymers having low chain-chain interactions as exemplified by polymers such as poly(dimethylsiloxane) and poly(4-methyl-1-pentene). Materials having high gas flux values typically possess low glass transition temperatures (Tg) because of low chain-chain interaction in the polymeric material. As a consequence, these materials typically require special processing conditions to build in chemical and/or physiochemical crosslinking if such materials are to be employed in other than low application temperatures. In contrast, polymers having strong chain-chain interactions typically have rather high Tg values and often exhibit rather low gas flux.
Polyimides, which generally have strong chain-chain interactions and high Tg values, have been reported to exhibit rather high gas flux values when certain structural characteristics are present. Specifically, U.S. Pat. No. 3,822,202 (1974); Re 30,351 (1980) discloses a process for separating fluids using a semi-permeable membrane made from polyimides, polyesters or polyamides. The repeating units of the main polymer chain which have at least one rigid divalent sub-unit, the two main chain single bonds extending therefrom which are not colinear, are sterically unable to rotate 360.degree. around at least one of these bonds, and has 50% or more of its main chain atoms as members of aromatic rings.
U.S. Pat. No. 4,705,540 discloses a highly permeable aromatic polyimide gas separation membrane and processes for using the membrane. The membrane comprises an aromatic polyimide membrane in which the phenylenediamines are rigid and are substituted on essentially all of the positions ortho to the amine substituents and the acid anhydride groups are essentially all attached to rigid aromatic moieties.
U.S. Pat. Nos. 4,717,393 and 4,717,394 teach polymeric membranes and processes for using such membranes for separating components of a gaseous mixture. The membranes disclosed in both of these patents are semi-flexible, aromatic polyimides prepared by polycondensation of dianhydrides with phenylenediamines having alkyl substituents on all positions ortho to the amine functions, or with mixtures of other non-alkylated diamines, some components having substituents on all positions ortho to the amine functions. Membranes formed from this class of polyimides are stated to exhibit improved environmental stability and gas permeability due to optimization of the molecular free volume in the polymer. Such membranes can also be photochemically crosslinked which in some instances results in a better performing semi-permeable membrane.
U.S. Pat. No. 4,378,400 discloses gas separation membranes formed from aromatic polyimides based upon biphenyltetracarboxylic dianhydride for separating various gaseous mixtures.
Attempts have also been made to synthesize membranes having high flux and high selectivity by creating a composite structure on the surface of the polymer by means of a chemical reaction between a labile polymer functionality and some "activating force". Such methods are taught in U.S. Pat. No. 4,657,564 wherein poly(1-trimethylsilylpropyne) is treated with a dilute fluorine gas stream and in U.S. Pat. No. 4,717,393 wherein a polyimide containing a benzophenone-containing linking group is irradiated with a medium pressure mercury lamp.
U.S. patent application Ser. No. 07/420,087, filed 10/11/89, teaches a class of polyimide membranes containing copolymerizable, surface modifiable units containing both aromatic diamines and alkenylated diamines having an allyl or allylaryl group preferably positioned ortho to an amine functionality. The polyimide membranes can be surface-modified by treatment with an activating force such as high energy electomagnetic irradiation or with a free radical source to impart high selectivity to the membrane without causing a large decrease in composite permeance.
A need in the art exists for improved polymeric materials that are highly permeable and which provide selective separation of various gaseous combinations. Such materials would especially be useful in commercial, non-cryogenic gas separation processes.